Conventional wide-range bidirectional resonant converters are generally classified into two types: two-stage structure, and one stage with online parameter change. In the former type, one additional conversion stage is dedicatedly added to extend the voltage gain range, while the original stage has little voltage regulation capability. In the latter type, multiple voltage gain ranges can be generated by changing the converter parameters, including resonant component parameters or transformer turn ratio.
FIG. 1 shows a topology of conventional bidirectional DC/DC resonant converter. As shown in FIG. 1, VH and VL are the voltages of the two DC ports of the converter. Due to the topological symmetry, the converter operations in the two power transferring directions are the same. To simplify the analysis, VH is the high voltage side, which is assumed as the DC bus side. VL is the low voltage side, which is assumed as the terminal voltage of the battery or supercapacitor. Thus VL usually has a wide operation range. Its maximum value VLmax can be above 2.5 times of the minimal value VLmin. VH has a relatively smaller variation, the ratio of VHmax to VHmax is usually below 1.5. When power is transferred from VH to VL, the converter is defined to work in forward mode, and vice versa.
In order to increase the voltage gain range in forward direction, full-bridge and half bridge operation modulations are both used. The full-bridge mode in forward direction is shown in FIG. 2A. Because the switches on the rectification side turn off with ZCS when the switching frequency fs is below the resonant frequency fr, only the waveforms in this range are shown. Ts is the switching cycle. Vab is the voltage across the midpoints of the two primary legs. N is the transformer turn ratio, and N=N2/N1. Vgs1˜Vgs8 are the gate signals of the eight switches. ir1 and ir2 are the primary and secondary resonant currents. The VL-side switches Sp5-Sp8 are not turned on, and only their antiparallel diodes are used. Sp1 and Sp4 have the same gate signal, and so do Sp2 and Sp3. The duty cycle of all the four switches is 0.5. Because the diagonal switches have the same gate signals, Vab is a bipolar square wave. The voltage gain is regulated by changing the switching frequency, which is Gff=N·gff. gff is the AC voltage gain regardless of the transformer turn ratio, which is controlled by the switching frequency. gff decreases as fs increases. When the converter works in backward direction, the modulation is the same, and the voltage gain from VL to VH is Gfb=1/N·gfb.
When VL becomes very low, instead of further increasing the switching frequency to extremity, the converter can work in half bridge mode to reduce the gain by roughly half. The half-bridge modulation in forward direction is shown in FIG. 2B. The operation of the converter is the same as that in the full bridge mode, except that the amplitude of the AC component of Vab is half of that in the full-bridge mode, VL becomes half at the same switching frequency and equivalent load. Thus the gain is indicated as Ghf=0.5N·ghf. The conversion gain in backward direction should be Ghb=2/N·ghb to match the two port voltages.
These converters exhibit characteristics of step-up during power transfer in the forward direction and characteristics of step-down in the backward direction, which would be problematic for some applications requiring step-up in the backward direction.
As regards electric vehicle charger for example, in a normal mode of operation, the converter works in a step-down mode to charge and maintain the voltage of a low voltage DC battery, e.g., a 48 volt DC battery, from the high voltage, e.g., 400 volt DC, available via the power grid. In an emergency mode, it would be desirable to have the DC-DC converter work in a step-up mode to boost the low voltage (48 volt) to a high voltage (400 volt) to feed the HVAC system. The conventional converters would not have supplied voltage in a backward direction at a higher level required by feeding the AC grid with the power provided by the low voltage DC battery.
In another example, both the DC battery and supercapacitor have a wide operational voltage range to be fully charged or discharged. The ratio of the highest and lowest terminal voltages can be above 2.5. Moreover, the bus voltage of the AC grid can also be variable for flexible power control. Therefore, it is desirable that the DC/DC converter may work over a wide voltage range in order to fully take advantage of the energy storage devices.
Using the DC-DC converter in step-up mode is a high-power operation. Accordingly, it is desired to have high conversion efficiency in step-up operation so as not to waste valuable energy. In contrast, the step-down operation can be high power (when the battery is empty and must be recharged rapidly) or low power (when the battery is less than fully charged). Accordingly, it would be desirable to enhance the low power efficiency of a bidirectional DC-DC converter with no added active or passive components and with no modification in high power operation and at no additional cost.